Tread design for tires remains very much an art rather than a science. Predicting the wear rates on tire treads is uncertain, necessitating testing of new tread designs. The different sections on the tread wear at different rates, dependant upon the design of the tread, the composition of the tire, inflation, road conditions, usage, and the camber and toe-in of the tire. Since theoretical prediction is inexact, testing is required.
This testing of tire treads is time consuming and expensive: a full road test requires a long time to complete. It is customary to carry out relatively short usage tests, and then extrapolate the relatively small amount of wear induced to predict long term tread wear. The testing of the tire, usually on a vehicle, either in a laboratory setting, on a test track, or on actual roads, is referred to as running the tire, or a test run, or a run.
It is difficult to accurately measure small amounts of tread wear. The tread groove depth is often measured with a potentiometer probe. Lasers are known to be employed for more accurate measurement, but even when lasers are used, the runs must still wear away enough of the tread to provide accurate measurements. The result is that road tests are time consuming. Such tests may last for 5,000 kilometers. The after run testing is itself laborious and time consuming, Also, the method is inherently unable to determine uneven wear within a section of tread.
A different method of testing wear is to apply adhesive tape to the tire surface and examine the damage to the tape after a short run. However, the tape may lose adhesion and fall off the tire, and even if it does not, the test does not provide useful quantitative data.
An improved method of tire wear measurement (GB2232492A, 1990, Sumitomo Rubber Ind, Ltd.) is to paint the surface of the tire. In this method, a layer of rubber-based paint is applied to the tread surface of the tire and allowed to dry or cured to obtain a strong bond. A second layer of a different color of paint is then applied on top of the first layer and also dried or cured. This process is repeated until the tread surface has at least five coats of paint on it. The tire is then run for a short distance, typically 15-50 kilometers. The tread surface is then examined and the different colors of paint visible are noted to help determine the rate of wear on different sections of the tire. Exposure of deeper layers indicates greater wear.
This method has a number of disadvantages. The data gathered is essentially qualitative, not quantitative, and thus is difficult to analyze. Typically, a chart is drawn up, often by hand, with the different areas which display different colors indicated. In addition, it is necessary to run the tire for sufficient time to wear away enough paint to make the results unequivocal, i.e. 15-50 kilometers. At least five different colors of paint are required. The thickness of the layers may vary without the knowledge of the tester, corrupting the results. A considerable amount of time is spent allowing one coat of paint to dry or cure before applying the next coat. Vulcanization is required in order to guarantee a good bond of the paint to the tire. The results are low in resolution. Finally, the boundary between layers is usually not precise. Rather, the region displaying two different colors of paint is usually a slow blend from one color to the other, making accurate measurement difficult.